\chapter*{Acknowledgement}
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My entire research, including the results presented in this dissertation would not have been possible without my supervisor Prof. Dr. Dorel Lucanu. I would like to especially thank him for his patience and his advices and also for the fact that he always knows how to find the best solution of any problem I have. His passion for research has a positive influence on my activity and increases my curiosity and desire to study.

Concerning research I would also like to thank Adrian Iftene and Ionu{\c t} Pistol which opened to me the research "door" and to my great research team: Ionu{\c t} Apetrei, Irina M{\u a}riuca As{\u a}voae, Mihai As{\u a}voae, Chucky Ellison, Gheorghe Grigora{\c s}, David Laz{\u a}r, Radu Mereu{\c t}{\u a}, Elena Naum, Grigore Ro{\c s}u, Traian Florin {\c S}erb{\u a}nu{\c t}{\u a}, and Andrei {\c S}tef{\u a}nescu. I want to thank all of them for their valuable support.

I would like to especially thank to my parents Gheorghi{\c t}{\u a} and Ani{\c s}oara and to my sister Andreea for their uninterrupted support in my everyday life. Each of them spent, in a way or another, a lot of effort to sustain me every time I needed.

Finally, I would like to thank all my professors from high-school, college, and master, which have, each of them, a small contribution to my education.
